Interior of the Earth Notes 7th Social Science

Interior of the Earth Notes 7th Social Science

7th Social Science Lesson 3 Notes in English

3. Interior of the Earth

1. Explain about our mighty Earth:

The earth, our homeland, is a dynamic planet. The earth’s surface has lofty mountains, high plateaus, large plains and deep valleys etc. The earth’s surface is constantly undergoing changes inside and outside.

2. Mention the layers of the Earth:

On the basis of the study of earthquake waves the spherical earth is found to be three concentric layers. They are:

• The crust
• The mantle
• The core.

3. Explain the Crust and its Thickness:

The crust is the outermost layer of the earth. Its average thickness varies from 5 to 30 km. It is about 35 km on the continental masses and only 5 km on the ocean floors.

4. What are Continental and Oceanic Crust?

Despite greater thickness, the continental crust is less dense than the oceanic crust because it is made of both light and dense rock types. The oceanic crust is composed mostly of dense rocks such as basalt.

5. Explain about Sial:

The upper part consists of granite rocks and forms the continents. It has the main mineral constituents are silica and alumina. So it is collectively referred to as Sial. It has an average density of 2.7g/cm3.

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6. Explain about Sima:

The lower part is a continuous zone of denser basaltic rocks forming the ocean floors, comprising mainly of silica and magnesium. It is therefore called Sima. It has an average density of 3.0g/cm3.

7. The sial and the sima together form the earth’s crust. Since the sial is lighter than the sima, the continents can be said to be ‘floating’ on a sea of denser sima.

8. What is The Mantle?

The next layer beneath the crust is called the mantle. It is separated from the crust by a boundary called Mohorovicic discontinuity. The mantle is about 2,900 km thick.

9. Explain the two parts of Mantle:

• The upper mantle with a density of 3.4 – 4.4g/cm3 extends down to 700 km
• The lower mantle having a density of 4.4– 5.5g/cm3 extends from 700 to 2,900 km.

10. What is the Core?

• The innermost layer of the earth is called the core. It is also known as barysphere.
• It is separated from the mantle by a boundary called Weichart-Gutenberg discontinuity.
• The central core has very high temperature and pressure. It extends from 5,150 km to 6,370 km. The average density of core is 13.0 g/cm3

11. Name the Various parts of Core:

• The outer core, which is rich in iron, is in liquid state. It extends between 2,900 – 5,150 km.
• The inner core, composed of Nickel and Ferrous (Nife), is solid in state.

12. What are Earth Movements?

• The lithosphere is broken into a number of plates known as the Lithospheric plates.
• Each plate, oceanic or continental moves independently over the asthenosphere.
• The movement of the Earth’s lithospheric plates is termed as tectonic movements.
• The energy required to move these plates is produced by the internal heat of the earth.

13. Define Exogenic and Endogenic Forces:

• The forces which act in the interior of the earth are called as Endogenic forces and the forces that work on the surface of the earth are called as Exogenic forces.
• Endogenic forces sometimes produce sudden movements and at the other times produce slow movements.

14. Define Earthquake:

A sudden movement of a portion of the earth’s crust which produces a shaking or trembling is known as an earthquake. Earthquakes may cause widespread damage to life and property.

15. What is Focus and Epicentre?

The point where these vibrations originate is called the focus of the earthquake. The point of the earth’s surface directly above the focus is called the epicentre of the earthquake.

16. Name the Instrument used to measure Earthquake:

The earthquake waves are recorded by an instrument known as seismograph. The magnitude of an earthquake is measured by the Richter scale. The numbers on this scale range from 0 to 9.

17. Causes of Earthquake:

• The chief cause of earthquake is the sudden slipping of the portion of the earth’s crust along fractures or faults.
• The movement of the molten rocks underneath the surface produce strains which break the rocks apart.
• The sudden shifting of landmass causes upheavals in the crust of the earth sending vibrations or waves into the surrounding portions of the earth.
• A violent or explosive eruption often causes the earth in its vicinity to quake. Earthquakes are often common in most volcanic areas.

18. Effects of Earthquakes:

• Earthquakes may cause changes in the earth’s surface. Vibrations often set landslides in mountainous regions.
• A greater danger in an earthquake is the falling of buildings.
• Most of the houses which collapsed were made of mud and bricks and proved to be death traps. Fire is another great danger.
• Underground water system is naturally disturbed by such movements.

19. How does the Earthquake Originates?

An earthquake which originates below or near the sea causes great disturbance in the water. The floods and waves cause great loss of life, sometimes more than the earthquake itself.

20. Define Tsunami:

Tsunami, a Japanese term, is the name given to the huge wave caused in the sea by an earthquake. Tsunamis are quite common along the coasts of Japan and other regions in the Pacific Ocean.

21. Distribution of Earthquakes:

• The world’s distribution of earthquakes coincide very closely with that of volcanoes.
• Regions of greatest seismicity are circum-Pacific areas, with the epicenters and the most frequent occurrences along the Pacific Ring of Fire. It is said that about 68 percent of earthquakes occur in this belt.
• Another 31 % of earthquakes take place in the Mediterranean-Himalayan belt including Asia Minor, the Himalayas and parts of north-west China.
• The remaining percent of earthquakes are occur in Northen Africa and Rift valley areas of the Red sea and Dead sea.

22. Where does that Major Earthquake occurs in India?

• In India, the Himalayan region and the Ganga-Brahamaputra valley are prone to earthquakes.
• A number of earthquakes have been experienced in this region. Some of them were very severe and caused extensive damage, e.g., the earthquake of Uttar Kashi in 1991 and Chamoli in 1999.
• The Deccan Plateau, which was supposed to be comparatively free from the dangers of the earthquakes, has experienced two severe earthquakes in the past, the Koyna earthquake in 1967 and the Latur earthquake in 1993.

23. What are Volcanoes?

A volcano is a vent or an opening in the earth’s crust through which hot magma erupts from deep below the surface. The opening is usually circular in form.

24. Name the Various Constituents of Volcanoes:

• The molten rock material within the earth, together with gases, is called magma. After it rises to the surface, it is called as lava.
• In course of time, lava and other materials flow out of a volcano accumulate around the opening and form a conical hill or a mountain vent is an openning or mouth of a volcano.

25. What is Caldera?

• The top of this cone is usually marked by a funnel-shaped depression, which is called a crater. If the crater of a volcano is of great size and is shaped like a basin, it is called a caldera.
• Calderas are caused by violent explosions which blow away entire tops of great cones.

26. What are the Causes of Volcanic Activity?

• The temperature increases as the depth increases at the rate of 1ºc for every 35 metres.
• There is also great pressure. At a depth of about 15 km the pressure is about 5 tonnes per cm^2of rock.
• Under these circumstances, the interior of the earth is in a semi-molten state called magma.

27. Why does the Volcanic Material Bursts?

The magma, under great pressure has the capacity to dissolve great volume of gas, some gases are also combustible. Th is makes volcanic material burst forth through the weak spots in the earth’s crust.

28. What is Fissure Eruption?

Sometimes, magma rises slowly to the surface and spreads over a vast area. This is known as fissure eruption. Some plateaus and plains have been formed in this way, e.g., Deccan Plateau in India and the Colombian Plateau in North America.

29. What is Explosive Eruption?

If the magma rises quickly to the surface, lava is thrown high into the atmosphere. Besides lava, ash, steam, gases and pieces of rocks are also thrown out. This type of eruption is known as explosive eruption. The terrible explosion on the island of Krakatoa (27th August 1883) in Indonesia is an example for explosive type of eruption.

30. Explain the Viscosity of Water and Honey:

Honey has high viscosity, so it flows slowly, whereas water has low viscosity, so it flows easily.

31. How the Viscosity of lava Determined?

The viscosity of lava is determined by the amount of silica and water in magma. Highly viscosity lava is rich in silica and has little water. Low viscosity lava has little silica, but a lot of water. It moves rapidly forming smooth flows.

32. What are the different Types of Volcanoes?

The shape of a volcano depends on the type of lava and the force of the eruption. On the basis of shape, there are three types of volcanoes. They are

• Shield volcano
• Cinder-cone volcano
• Composite volcano

33. What is a Shield volcano?

A shield volcano is formed by quiet eruption of lava with a low silica content. Such a volcano has a wide base and a cone with gentle slopes. Volcanoes of the Hawaii islands are of this type.

34. What is a Cinder-cone volcano?

Silica-rich magma traps gases inside the volcano until enough pressure is built to push the magma out of the earth’s crust. They are known as cinder-cone volcanoes. Many volcanoes of Mexico and Central America belong to this group.

35. What is a Composite volcano?

Composite volcanoes are made of alternate layers of lava, cinder and ash. They are also called strato volcano. St.  Helens is an example of composite volcano.

36. How volcanoes are grouped according to their periodicity?

Volcanoes are also grouped according to their periodicity of eruptions such as active, dormant and extinct.

37. What are Active Volcanoes?

Active Volcanoes that erupt frequently are called active volcanoes. Most of the active volcanoes lie in the Pacific Ring of Fire belt which lies along the Pacific coast.

38. There are about 600 active volcanoes in the world, such as Mt. Stromboli in Mediterranean Sea, St.Helens in USA, Pinatubo in Philippines. Mauna Loa in Hawaii (3,255m.) is the world’s biggest active volcano.

39. What are Dormant Volcanoes?

Dormant volcanoes have shown no sign of activity for many years but they may become active at any time. It is also called Sleeping Volcano Vesuvius mountain of Italy, Mt Fujiyama in Japan, Mt. Krakatoa of Indonesia are famous examples of this types.

40. What are Extinct Volcanoes?

The top of extinct volcanic mountains have been eroded.  Mt Popa of Myanmar and Mt. Kilimanjaro and Mt. Kenya of Africa are examples of extinct volcanoes.

41. Distribution of Volcanoes in the world:

Volcanoes are located in a clearly-defined pattern around the world. They are closely related to regions that have been intensely folded or faulted. There are about 500 active volcanoes and thousands of dormant and extinct ones.

42. Where are the Places of Major Volcano occurence?

• They occur along the coastal mountain ranges, as off-shore islands and in the midst of oceans, but there are a few in the interior of continents.
• The volcanic belts are also the principal earthquake belts of the world.

43. Name the three Belts of Volcanic Activity:

• The Circum – Pacific belt
• The Mid continental belt
• The Mid Atlantic belt

44. What is Circum Belt?

• This is the volcanic zone of the convergent oceanic plate boundary. It includes the volcanoes of the eastern and western coastal areas of Pacific Ocean.
• This zone is popularly termed as the Pacific Ring of Fire which has been estimated to include two-thirds of the world’s volcanoes.

45. What is Mid continental belt?

• This is the volcanic zone of convergent continental plate boundaries that includes the volcanoes of Alpine mountain chains, the Mediterranean Sea and the fault zone of eastern Africa.
• The important volcanoes are Vesuvius, Stromboli, Etna, Kilimanjaro and Kenya. Surprisingly, the Himalayas have no active volcanoes at all.

46. What is Mid Atlantic Belt?

This belt represents the divergent boundary of plates located along the mid Atlantic ridges. Volcanoes of this area are mainly of fissure eruption type.

47. Various Examples of Mid-Atlantic Belt:

Iceland is the most active volcanic area and is located on the mid-Atlantic ridge. St. Helena and Azores Island are other examples.

48. Earth is called as blue Planet. 71% of the earth is covered by water.

49. The crust forms only 1 percent of the volume of the earth, 84 % consists of the mantle and 15 % makes the core.

50. What is the Radius of the earth?

The radius of the earth is 6371 km.

51. What is Asthenosphere?

The asthenosphere is the part of the mantle that flows and moves the plates of the earth.

52. An earthquake of 2.0 on Richter scale or less can be felt only a little. An earthquake over 5.0 on Richter scale can cause damage from things falling. A 6.0 on Richter scale or higher magnitude is considered very strong and 7.0 on Richter scale is classified as a major earthquake.

53. Name the three types of earthquake waves:

• P waves or longitudinal waves
• S waves or transverse waves
• L waves or surface waves

54. On 26th Dec 2004 Tsunami in the Indian Ocean swept coastal area of Indonesia, India, Srilanka, Thailand etc., They caused immense damage to life and property in the coastal area

55. What is Volcanology?

The scientific study of volcanoes are called volcanology. People who study valcanoes are called volcanologists.

56. What are Barren Island?

Barren island is situated in the Andaman Sea, and lies about 138 km northeast of the territory’s capital. It is only in active volcano along the chain from sumatra to myanmar. Last eruption occurred in 2017.

57. Stramboli is known as the ‘light house of Mediterranean sea’.